The present invention generally relates to the field of radio telecommunications. More particularly, the present invention relates to downlink power level control in a time division multiple access (TDMA) based radio telecommunications system.
In a telecommunications system, e.g., a cellular radio system, any one of several access strategies may be employed, for example, frequency division multiple access (FDMA), code division multiple access (CDMA), or time division multiple access (TDMA).
In North America, a digital cellular radiotelephone system using TDMA is called the Digital Advanced Mobile Phone System (D-AMPS), some of the characteristics of which are specified in the TIA/EIA/IS-136 standard published by the Telecommunications Industry Association and Electronic Industries Association (TIA/EIA). Another digital communication system, using direct sequence CDMA, is specified by the TIA/EIA/IS-95 standard. There are also frequency hopping TDMA and CDMA communication systems, one of which is specified by the EIA SP 3389 standard (PCS 1900). The PCS 1900 standard is an implementation of the GSM system, which is common outside North America, that has been introduced for personal communication services (PCS) systems.
In an FDMA based system, the frequency spectrum is divided into a number of disjunctive frequency bands, where each band serves as a separate radio channel. In a system that employs CDMA, different modulation or spreading codes are used to distinguish the various radio channels.
In a TDMA based system, however, the time domain is divided into time frames. Each time frame is then further divided into a number of timeslots, for example, three timeslots. Thus, each carrier frequency-timeslot combination constitutes a different physical channel over which a communications signal burst can be transmitted. In a cellular radio telecommunications system, a communications signal burst transmitted from a mobile station to a corresponding radio base station is referred to as an uplink burst. In contrast, a communications signal burst transmitted from the radio base station to the mobile station is referred to as a downlink burst.
FIG. 1 illustrates a conventional TDMA cellular radio system including cells C1-C10 and base stations B1-B10, one base station per cell. The base stations are typically situated in the vicinity of the cell center and have omnidirectional antennas. The base stations of adjacent cells may, however, be colocated in the vicinity of cell borders and have directional antennas, as is well known to those skilled in the art.
The system also includes mobile stations M1-M10 that are movable within a cell and from one cell to another. A mobile switching center (MSC) is connected to the base stations by, e.g., cables or fixed radio links. The MSC is also connected to a fixed public switching telephone network or a similar fixed network with ISDN facilities. In addition to the MSC illustrated in FIG. 1, there may also be other mobile switching centers.
Power control, i.e., the ability to modify or adjust the power levels associated with communications signal bursts, particularly, downlink communications signal bursts transmitted from a base station to a mobile station, is important in a telecommunications system to insure that the signal quality associated with a given channel is adequate. Power control also helps improve the spectral efficiency of the system as a whole by balancing average, system-wide signal quality and system capacity and effectively limiting the emitted energy that acts as interference at other mobile.
Downlink power control relies on received signal quality and received signal strength as reported from the mobile station in order to regulate the base station output power so that minimum requirements for speech quality are fulfilled, but energy emitted is minimized to keep interference low. For this purpose, algorithms are implemented in the base stations, which use parameters transmitted from the mobile station, e.g., the measured quality and the measured RSSI of the downlink data, and radio network management parameters transmitted from the MSC, e.g., acceptable speech quality. A more detailed description of downlink power control is provided in U.S. Pat. No. 6,529,494, issued Mar. 4, 2003, and herein Incorporated by reference.
In accordance with the present TDMA standard, TIA/EIA 136 Rev. A, downlink transmission power level remains constant throughout each time frame. Thus, a mobile station receiving a downlink burst during a given timeslot expects the power level of the received burst to remain constant, or nearly constant, over the timeslot, not withstanding attenuations due to fading. However, it is highly probable that the TDMA standard will soon incorporate downlink power control, where the transmission power level from timeslot to timeslot may be adjusted, to achieve better signal quality and spectral efficiency, as stated above.
Downlink power control must be compatible with the handoff procedure used for the cellular system, e.g., Mobile Assisted Handoff (MAHO). According to this procedure, the MSC orders the mobile station to continuously make signal strength measurements on channels other than the channel assigned to the mobile station. This is done by transmitting a MAHO list to the mobile station via the base station, as the communication is set up. The list is defined for each cell and typically includes control channels of neighboring cells. In a TDMA system, the MAHO list may include, e.g., 24 or 12 different channels. The mobile station makes the measurements in a time period not used for communication and reports the measurements back to the base station, which transmits the information to the MSC. Upon a handoff request, the MSC chooses the handoff target cell based on the control channel with the highest signal strength. Other uses for the MAHO list have been suggested, as in WO 98/30043 and WO 97/31502.
One problem with implementing downlink power control in TDMA based systems is that, for handoff purposes, mobile stations are required to measure, and thereafter report, approximately once every second, received signal strength information (RSSI), as one skilled in the art will readily appreciate. Due to the fact that the current TDMA standard maintains transmission power level constant throughout each time frame for a given carrier frequency, and because the TDMA standard does not define when or how RSSI is to be measured, some mobile stations are designed to measure RSSI during the timeslot in which they are receiving downlink data, while other mobile stations are designed to measure RSSI during adjacent timeslots. Moreover, some mobile stations are designed such that they sample received signal strength during a portion of a timeslot, while other mobile stations sample received signal strength over the entire timeslot, and therefrom, calculate an average signal strength value. If the TDMA standard incorporates downlink power control, mobile stations which are designed in accordance with the present TDMA standards (i.e., legacy mobile stations), particularly those mobile stations that are not designed to measure RSSI during the timeslot in which they are receiving downlink data, may be unable to accurately measure and report RSSI, because the RSSI measured in timeslots not dedicated to the mobile station may not be a good indicator of the actual RSSI in the timeslot dedicated to the mobile station.
Also, if downlink power control in each timeslot is introduced, the differences in measuring RSSI may become a problem. Mobile stations that report similar signal strengths in timeslots in which downlink power control is not used will report different signal strengths timeslots in which downlink power control is used. This could make the downlink power control functionality difficult to use.
Accordingly, it would be highly desirable to provide a technique whereby any legacy mobile station is capable of measuring and reporting RSSI accurately while operating in a TDMA system that employs downlink power control.
It is therefore an object of the present invention to provide a technique for accurately measuring and reporting RSSI.
According to exemplary embodiments, this and other objects are met by a method and system, in a Time Division Multiple Access based radio telecommunications system that employs downlink power control, of measuring and reporting received signal strength information. A downlink data signal is transmitted from a base station to a mobile station during a first timeslot. A control channel signal or a dedicated traffic channel signal with an equivalent output power behavior as the control channel signal is transmitted from the base station throughout a cell in which the mobile station is operating. At the mobile station, a received signal strength of the control channel signal or the dedicated traffic channel signal is measured. The received signal strength as measured by the mobile station is reported to the base station. The measured received signal strength is compensated for to obtain signal strength information associated with the downlink data signal received by the mobile station during the first timeslot.
The measured received signal strength may be compensated for by subtracting an attenuation level associated with the downlink data signal from the received signal strength measurement associated with the control channel signal or the dedicated traffic channel signal. Alternately, the difference in output power between the control channel signal or the dedicated traffic channel signal and the downlink data signal may be added to the received signal strength measurement associated with the control channel signal or the dedicated traffic channel signal. If different antenna systems are used for transmitting the control channel signal and the downlink data signal, different antenna system gains may be compensated for. If different frequency bands are used for transmitting the downlink data signal and the control channel signal, different propagations at the different frequencies may be compensated for.
The received signal strength of the control channel signal or the dedicated traffic channel signal may be measured during a period between when the mobile station transmits and receives or during a period between when the mobile station receives and transmits. The received signal strength may be measured during the first timeslot or during a timeslot other than the first timeslot. The received signal strength may also be measured during a time period associated with Mobile Assisted Handover (MAHO) measurements, MAHO measurements being performed for channels in a MAHO list that includes the base station""s control channel and/or the dedicated traffic channel.